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Register a new userDetectability of 21cm-signal during the Epoch of Reionization with 21cm-Lyman-{alpha} emitter cross-correlation. II. Foreground contamination
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Shintaro Yoshiura Mr.
Publication date
2017
fields
Physics
and research's language is
English
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Cross-correlation between the redshifted 21 cm signal and Lyman-{alpha} emitters (LAEs) is powerful tool to probe the Epoch of Reionization (EoR). Although the cross-power spectrum (PS) has an advantage of not correlating with foregrounds much brighter than the 21 cm signal, the galactic and extra-galactic foregrounds prevent detection since they contribute to the variance of the cross PS. Therefore, strategies for mitigating foregrounds are required. In this work, we study the impact of foreground avoidance on the measurement of the 21 cm-LAE cross-correlation. We then simulate the 21 cm observation as observed by the Murchison Widefield Array (MWA). The point source foreground is modelled from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey catalogue, and the diffuse foreground is evaluated using a parametric model. For LAE observations, we assume a large survey of the Subaru Hyper Supreme-Cam (HSC), with spectroscopic observations of the Prime Focus Spectrograph (PFS). To predict the 21 cm signal, we employ a numerical simulation combining post processed radiative transfer and radiation hydrodynamics. Using these models, the signal-to-noise ratio of 2D PS shows the foreground contamination dominates the error of cross-PS even in the so-called `EoR window. We find that at least 99% of the point source foreground and 80% of the galactic diffuse foreground must be removed to measure the EoR signal at large scales $k<0.5 h rm Mpc^{-1}$. Additionally, a sensitivity 80 times larger than that of the MWA operating with 128 tiles and 99% of the point source foreground removal are required for a detection at small scales.
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Detection of the redshifted 21cm-line signal from neutral hydrogen in the intergalactic medium (IGM) during the Epoch of Reionization (EoR) is complicated by intense foregrounds such as galactic synchrotron and extragalactic radio galaxies. The 21cm-Lyman-$alpha$ emitter(LAE) cross-correlation is one of the tools available to reduce the foreground effects because the foreground emission from such radio sources is statistically independent of LAE distribution. LAE surveys during the EoR at redshifts $z=6.6$ and $7.3$ are ongoing by the Subaru Hyper Suprime-Cam (HSC). Additionally, Prime Focus Spectrograph (PFS) will provide precise redshift information of the LAEs discovered by the HSC survey. In this paper, we investigate the detectability of the 21cm signal with the 21cm-LAE cross-correlation by using our improved reionization simulations. We also focus on the error budget and evaluate it quantitatively in order to consider a strategy to improve the signal-to-noise ratio. In addition, we explore an expansion of the LAE survey to suggest optimal survey parameters and show a potential to measure a characteristic size of ionized bubbles via the turnover scale of the cross-power spectrum. As a result, we find that the Murchison Widefield Array (MWA) has ability to detect the cross-power spectrum signal on large scales by combining LAE Deep field survey of HSC. We also show that the sensitivity is improved dramatically at small scales by adding redshift information from the PFS measurements. The Square Kilometre Array (SKA) has a potential to measure the turnover scale with an accuracy of $6times10^{-3}~{rm Mpc^{-1}}$.
Detecting $rm H_I$ 21cm line in the intergalactic medium (IGM) during the Epoch of Reionization (EoR) suffers from foreground contamination such as Galactic synchrotron and extragalactic radio sources. Cross-correlation between the 21cm line and Lyman-$alpha$ emitter (LAE) galaxies is a powerful tool to identify the 21cm signal since the 21cm line emission has correlation with LAEs while the LAEs are statistically independent of the foregrounds. So far, the detectability of 21cm-LAE cross-power spectrum has been investigated with simple LAE models where the observed Ly$alpha$ luminosity is proportional to the dark matter halo mass. However, the previous models were inconsistent with the latest observational data of LAEs obtained with Subaru/Hyper Suprime-Cam (HSC). Here, we revisit the detectability of 21cm-LAE cross-power spectrum adopting a state-of-the-art LAE model consistent with all Subaru/HSC observations such as the Ly$alpha$ luminosity function, LAE angular auto-correlation, and the LAE fractions in the continuum selected galaxies. We find that resultant cross-power spectrum with the updated LAE model is reduced at small scales ($ksim 1 rm Mpc^{-1}$) compared to the simple models, while the amplitudes at large scales ($k lesssim 0.2 rm Mpc^{-1}$) are not affected so much. We conclude that the large-scale signal would be detectable with Square Kilometre Array (SKA) and HSC LAE cross-correlation but detecting the small scale signal would require an extended HSC LAE survey with an area of $sim 75 rm deg^2$ or 3000 hrs observation time of 21cm line with SKA.
A detection of the predicted anticorrelation between 21cm and either Ly-alpha or H-alpha from the Epoch of Reionization (EOR) would be a powerful probe of the first galaxies. While 3D intensity maps isolate foregrounds in low k_parallel modes, infrared surveys cannot yet match the field of view and redshift resolution of radio intensity mapping experiments. In contrast, 2D (i.e., broad band) infrared intensity maps can be measured with current experiments and are limited by foregrounds instead of photon or thermal noise. We show 2D experiments can measure most of the 3D fluctuation power at k<0.2 Mpc^-1 while preserving its correlation properties. However, we show foregrounds pose two challenges: (1) simple geometric effects produce percent-level correlations between radio and infrared fluxes, even if their luminosities are uncorrelated; and (2) radio and infrared foreground residuals contribute sample variance noise to the cross spectrum. The first challenge demands better foreground masking and subtraction, while the second demands large fields of view to average away uncorrelated radio and infrared power. Using radio observations from the Murchison Widefield Array and near-infrared observations from the Asteroid Terrestrial-impact Last Alert System, we set an upper limit on residual foregrounds of the 21cm--Ly-alpha cross power spectrum at zsim7 of Delta^2<181 kJy/sr * mK (95%) at ellsim800. We predict levels of foreground correlation and sample variance noise in future experiments, showing that higher resolution surveys such as LOFAR, SKA-LOW, and the Dark Energy Survey can start to probe models of the 21cm--Lyalpha EOR cross spectrum.
An obstacle to the detection of redshifted 21cm emission from the epoch of reionization (EoR) is the presence of foregrounds which exceed the cosmological signal in intensity by orders of magnitude. We argue that in principle it would be better to fit the foregrounds non-parametrically - allowing the data to determine their shape - rather than selecting some functional form in advance and then fitting its parameters. Non-parametric fits often suffer from other problems, however. We discuss these before suggesting a non-parametric method, Wp smoothing, which seems to avoid some of them. After outlining the principles of Wp smoothing we describe an algorithm used to implement it. We then apply Wp smoothing to a synthetic data cube for the LOFAR EoR experiment. The performance of Wp smoothing, measured by the extent to which it is able to recover the variance of the cosmological signal and to which it avoids leakage of power from the foregrounds, is compared to that of a parametric fit, and to another non-parametric method (smoothing splines). We find that Wp smoothing is superior to smoothing splines for our application, and is competitive with parametric methods even though in the latter case we may choose the functional form of the fit with advance knowledge of the simulated foregrounds. Finally, we discuss how the quality of the fit is affected by the frequency resolution and range, by the characteristics of the cosmological signal and by edge effects.
The cross-correlation between fluctuations in the electron scattering optical depth $tau_{rm es}$ as probed by future Cosmic Microwave Background (CMB) experiments, and fluctuations in the 21cm differential brightness temperature $Delta T_{rm 21cm}$ as probed by ground-based radio interferometers, will trace the reionization history of the Universe. In particular, the $tau_{rm es}-$21cm cross-correlation should yield a determination of the characteristic bubble size distribution and ionization fraction as a function of redshift. When assuming that the cross-correlation signal is limited by instrumental noise rather than by foregrounds, we estimate its potential detectability by upcoming experiments. Specifically, the combination of HERA and Simons Observatory, CMB-S4 and PICO should yield a signal-to-noise ratio around 3 - 6, while and the exploitation of the SKA should increase it to 10-20. Finally, we have discussed how such levels of detectability can be affected when (simply modeled) 21cm foregrounds are present. For the most promising PICO$times$SKA configuration, an efficiency of foreground removal to a level of $7times 10^{-4}$ is needed to achieve a $5sigma$ detection of the cross-correlation signal; in addition, safe avoidance of foreground contamination in the line-of-sight Fourier modes above $0.03 ,h,rm Mpc^{-1}$ would guarantee a detection significance around $3sigma$.
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